This invention relates generally to paper making machines and, more particularly, to press rolls and presses formed thereby.
The most common conventional method for removing water from paper and cardboard webs is to pass the web through a press nip formed by a pair of rolls situated opposite one another. It is also conventional to pass one or two press fabrics through the dewatering press nip, the fabrics carrying the water removed from the web and, moreover, carrying the web onwards.
As the rates of production of paper machines have increased, the web dewatering performed by press nip dewatering methods has become a factor limiting further increases in production rates due to the fact that the press nips formed by a pair of conventional press rolls are relatively short or narrow. It therefore follows that at high production speeds the web remains in such press nips for only a relatively short time. On the other hand, since the fibrous structure of the web inherently results in a certain flow resistance, the water in the web requires a certain amount of time to be able to escape from the web into the hollow face of a press roll and/or into a press fabric.
Attempts have been made to increase the dewatering output in nip presses by increasing the nip pressure. However, it has been found that at a certain line pressure a limit is reached beyond which further increases in the nip pressure are no longer practical because the structure of the web can no longer withstand the high compression forces.
It is also possible to increase the area of a press nip formed by a pair of rolls by using rolls having relatively large diameters and/or by passing relatively soft press fabrics through the nip. However, a limit is eventually reached in even these arrangements beyond which any increase in dewatering becomes uneconomical.
Presses are also known in the art which consist of a pair as rolls having resilient coatings such, for example, of rubber or polyurethane, for the purpose of extending or widening the nip area. For example, reference is made to U.S. Pat. No. 3,535,760 which discloses a press which includes a roll having an elastic coating. It is also known to provide rolls having elastic coatings with various groove formations in order to increase their elasticity. In this connection, reference is made to U.S. Pat. No. 3,630,837.
It is also known in the art to pass a resilient mat through a nip in order to lengthen or widen the press nip.
A serious drawback of presses having a construction as described above wherein a resilient or elastic roll or pair of rolls or a resilient mat are utilized is that a so-called "rewetting" phenomenon occurs when a web is passed through such a press. Indeed, rewetting occurs even in the case of dewatering a web in a conventional nip press. For example, a web will be situated for about 20 ms within an area of the mid-portion of a press nip in which the compression pressure has been reduced to a level which is below a certain limit. Despite the relatively short duration of time during which the web is present in this region of the press nip, a rewetting of about 30% will occur. This phenomenon represents a fundamental drawback of conventional press nips which has limited the dewatering capacity even of nips provided with relatively wide press zones.